Evolution structurale d'oxydes isomorphes MeX2O4: Relation entre dilatation,vibrations et rigidite

The neutron diffraction studies at low temperatures of the isomorphic compounds MeX₂O₄ (Pb₃O₄, SnPb₂O₄, NiSb₂O₄, ZnSb₂O₄) are discussed and correlated. Only powdered samples were used. The various thermal expansion tensors (α_a, α_c) of these tetragonal structures are interpreted from the thermal expansion of some MeO or XO bonds, the values of which are compared to simulated values. The relation between the temperature factor $\text{B}$ and the volume V gives the mean elastic constant. From elementary hypothesis, the compressibility is evaluated for each compound with a mean calculated value $\text{χ}\text{= 1.8 ×}$ 10^?11 Pa^?1. The Grüneisen parameters γ might vary from 0.34 to 0.65. From the crystallographic data, the Debye temperatures are evaluated and the anisotropic rigidity is discussed. Knowing χ it is possible to evaluate the s_ij elastic constants indirectly and so to interpret the anisotropic thermal expansion.     

Lattice systems with a continuous symmetry

We investigate a continuous Ising system on a lattice, equivalently an anharmonic crystal, with interactions: $$\sum\limits_{\left\langle {x,y} \right\rangle } {\left( {\phi _x - \phi _y } \right)} ^2 + \lambda \left( {\phi _x - \phi _y } \right)^4 , \phi _x \in \mathbb{R}, x \in \mathbb{Z}^d .$$ We prove that the perturbation expansion for the free energy and for the correlation functions is asymptotic about λ=0, despite the fact that the reference system (λ=0) does not cluster exponentially. The results can be extended to more general systems of this type, e.g. an even polynomial semibounded from below instead of a quartic interaction. By a suitable scaling, λ corresponds to the temperature.     

Investigation of the particle-laden airflow generated by a pseudo-machining operation

A test rig recreating a typical machining induced gas–solid flow is used to investigate the behaviour of the emitted pollutant particles and their effect upon the airflow around the machine tool. The flow is driven by a rotating cylinder and a tangential jet of inertial solid particles. Experimental data concerning both particle flow and air flow are collected by means of phase Doppler particle analyser (PDPA) and particle image velocimetry (PIV). After describing the specific treatments used to discriminate the two phases, complete results are provided and discussed. Additionally, with the main objective being to optimize the design of pollutant capture devices for machining systems, tentative numerical simulations are carried out and compared to experimental data. For the one-phase case (air flow without particles), good agreement between simulations and experiments is found and the superiority of a wall-function based large eddy simulation (LES) over realizable k–ε$k''–''\epsilon$ modelling is highlighted. For two-phase cases, combination of LES and Lagrangian tracking with two-way coupling leads to simulation results that are reasonably accurate considering the low degree of modelling and the empiricism involved. Particle-to-particle collisions, disregarded in simulations, appear to be a predominant phenomenon in the jet source region, thus partly explaining some discrepancies observed between simulations and experiments.     

Mise en evidence et localisation des protons dans les bioxydes de plomb PbO2α et β chimiques et électrochimiques

Chemical and electrochemical samples of lead dioxides PbO₂α (orthorhombic) and PbO₂β (tetragonal) are studied by quasi-elastic neutron scattering and by X-ray diffraction between 77 and 550 K. The presence of OH^?-like proton species is established: the electrochemical species are characterized by two types of local proton motions ($\text{ΔE}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 300 μ}\text{eV}\text{, ΔE}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 40 μ}\text{eV}$). The thermal expansion of the chemical and electrochemical sample shows: (1) the loss of OH^?-like charged species after heating to 550 K, (2) an anisotropy of the OH ··· O bonds in the [PbO₆] octahedra chains which are oriented differently in the α and β structures. An interpretation of the role of protons is proposed.     

Les antimonites antiferromagnetiques MnSb2O4 et NiSb2O4

Neutron diffraction has been used to study the variation of antiferromagnetic order in the antimony isomorphous MnSb₂O₄ (T_N ～ 60 K) and NiSb₂O₄ (T_N ～ 46 K). The magnetic moments have been related to the Mn²⁺ and Ni²⁺ spins and magnetostrictive effects have been interpreted. The influence of the method of synthesis is mentioned: polycrystalline MnSb₂O₄ has been obtained from hydrothermal synthesis. Orthorhombic distortions are not connected with magnetic interactions but with structural defects.     

Infrared bounds and the peierls argument in two dimensions

We propose a definition of contours for spin systems which leads to improved estimates on the region of parameters where several phases coexist. We discuss as examples anisotropic rotators and a ⁴ lattice field theory. Our contours are estimated using infrared bounds and they are related to those of Euclidean Field Theory.Part of this work was done while Jean Bricmont was at the Mathematics Department of Princeton University and Jean-Raymond Fontaine was at the Mathematics Department of Rutgers University. This work was supported by NSF Grant No. MCS 78-01885 at Princeton University and by NSF Grant No. PHY 78-15920 at Rutgers University     

Electrical behaviour of catalytic nanostructured CeO2/CuOx composites under air-methane gas impulses

Nanostructured composites based on copper oxide and cerium dioxide phases [CuO-CeO₂] were elaborated from sol-gel route, with weight fractions of CuO phase ranging between 0 and 0.4. They are interesting potential catalysts allowing conversion of CH₄ and CO into CO₂ and H₂O and might be used in miniaturized gas sensors. An electrical study of this nanostructured system was carried out to determine catalytic behaviours under air-methane impulses at 350 °C. The electrical analysis was based on a specific homemade electronic device. Time dependent interactions between gas pulses and solid catalyst (CuO/CeO₂) were analyzed from a frequency modification of the electronic device. Kinetic parameters were determined from a model describing adsorption and desorption of gases adapted to short interaction time between gas and solid. These time dependent electrical behaviours were then correlated with infrared spectroscopy analyses allowing time dependent analysis of methane conversion into CO₂ gas, for long interaction time between gas and solid.     

Effects of lutetium doping on the X-ray-excited luminescence properties of the tungstate Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Lu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>WO<Subscript>4</Subscript>

Polycrystalline samples in the lutetium-doped zinc tungstate system Zn_1?x Lu _x WO₄ with 0 ≤ x ≤ 0.08 were synthesized using the coprecipitation method followed by thermal treatment at 1000 °C during 4 h. The polycrystalline samples were characterized by X-ray diffraction analysis, scanning electron microscopy (SEM), infrared spectroscopy, and luminescence analysis under X-ray excitation. Rietveld analyses were performed. The variation of the wolframite structure cell parameters in the range 0 ≤ x ≤ 0.05 were congruent with substitution of Zn²⁺ by Lu³⁺. SEM micrographs of the obtained samples presented improved crystallization with morphology depending on the lutetium fraction. The luminescence spectra obtained under X-ray excitation (E < 40 keV) were in the blue–green region, and their intensity increased with x up to x = 0.05. The differences in the intensities of the X-ray luminescence spectra could be related to additional cation vacancies resulting from substitution of Zn²⁺ by Lu³⁺.